1. Field of the Invention
The present invention relates to a method for manufacturing reduced iron pellets.
2. Description of the Related Art
A method for manufacturing reduced iron briquettes in Japanese Unexamined Patent Application Publication No. 6-316718 discloses a process for cooling the hot reduced iron briquettes. In the unexamined patent mentioned above, the reduced iron briquettes are slowly cooled to a temperature range of 350.degree. C. to 250.degree. C., by spraying water at a cooling rate between 150.degree. C./min and 250.degree. C./min, and the briquettes are then quenched by using cooling water. Another process in the unexamined patent is slowly cooling the reduced iron briquettes to a temperature range of 350.degree. C. to 250.degree. C., by using gas at a cooling rate between 150.degree. C./min and 250.degree. C./min, and the briquettes are then quenched using cooling water. Still another process in this unexamined patent is slowly cooling the reduced iron briquettes to 620.degree. C. to 550.degree. C. by using an inert gas, followed by spraying water to cool the briquettes to a temperature range of 350.degree. C. to 250.degree. C. at a cooling rate between 150.degree. C./min and 250.degree. C./min, and the briquettes are then quenched by cooling water.
Japanese Unexamined Patent Application Publication No. 10-158710 discloses a method for manufacturing reduced iron pellets by cooling hot reduced iron pellets discharged from a rotary kiln. According to the unexamined patent mentioned above, the reduced iron pellets are slowly cooled to 600.degree. C. at a cooling rate of not more than 500.degree. C./min. Through the cooling process described above, reoxidized films are formed on the surfaces of the reduced iron pellets. Since the reoxidized films are very dense and prevent the reduced iron pellets from being penetrated by moisture and oxygen, the reoxidized films can prevent reoxidation of the reduced iron pellets when they are stored for a long period of time. However, it is difficult to form the reoxidized films on the surfaces of reduced iron pellets discharged from rotary kilns only by air cooling.
When the reduced iron is processed to form briquettes such as those disclosed in Japanese Unexamined Patent Application Publication No. 6-316718, there are problems in the high expensive cost of providing facilities for processing briquettes and the high operating costs thereof. Therefore, it is preferable to cool the reduced iron and recover it in the form of pellets. However, since reduced iron pellets differ from reduced iron briquettes in size and characteristics, the method disclosed in Japanese Unexamined Patent Application Publication No. 6-316718 cannot be applied to pellets as written for briquettes.
In addition, when hot reduced iron pellets are quenched by cooling water, crushing strength thereof decreases by approximately 10 to 20 kg/cm.sup.2 ; however, decrease in degree of metallization thereof is not significant. A method for cooling the reduced iron pellets received in containers, such as a hopper, by using nitrogen or the like, has the problem in that the cooling facilities are expensive.